A good development move.
6... f6g4 Carlsen complicates the game. The knight supports the …e7-e5 break and can go to h6 if attacked. 7. e1d3  7... c8f5  8. f2f3  8... g4h6 The bishop on f5 is in the knight's way and it's not easy to break on e5 anymore. One idea that Black has in mind is to capture the knight on d3 and play Nf5 next.